High hardness material working method and working apparatus

ABSTRACT

The present invention provides a method of working a high hardness material including the steps of heating the high hardness material, working the high hardness material by using a polishing tool, cooling the polishing tool at a position which is different from a worked part, and washing the polishing tool at a position which is different from the worked part.

BACKGROUND ART

The present invention relates to a working method of a high hardness material, in particular relates to a working method suitable for surface polishing etc. of an SiC single crystal wafer.

In the past, a working method of a wafer which cuts an ingot to form a wafer and polishes the surface of the wafer to flatten it has been known. For example, when using a wafer comprised of SiC or other high hardness material for working, since the high hardness material has a large working resistance, to satisfy a predetermined working precision, working at a low working speed is necessary. As a result, the efficiency of working becomes low.

When polishing the surface of a high hardness wafer etc. for flattening, it is known to form a modified layer of a low hardness at the surface layer part of the wafer to thereby improve the working efficiency.

Japanese Patent Publication No. 2011-249449 A1 discloses a working method of a wafer which makes the wafer rotate while applying heat to the part to be polished to form a modified layer and using a polishing tool to polish the part of the wafer to which heat is applied.

CITATIONS LIST Patent Literature

-   PLT 1. Japanese Patent Publication No. 2011-249449 A1

SUMMARY OF INVENTION

In the method of heating and working a worked part of a high hardness material, the following problems arose.

The first is the problem of the heat resistance of the grindstone. Working a high hardness material has to be performed by an abrasive of at least the hardness of the worked material. For example, when the worked material is SiC, use of a diamond abrasive or cBN abrasive becomes necessary. When a diamond abrasive becomes 600° C. or more or a cBN abrasive becomes 1280° C. or more, the hardness falls, so to realize a process of heating a worked part of a high hardness material for working, it is necessary to reduce the heat input to the cutting tool.

The second is the problem of the drop in working due to clogging due to the dry process. Normally, in cutting work, to prevent a drop in the workability due to clogging of the grindstone by swarf, a working solution is fed while working to cause the swarf to be expelled. However, in the method of heating a worked part of a high hardness material for working, to heat the worked part to 1200° C. or so and stabilize the temperature, it is necessary to perform the working by a dry process. For this reason, the working solution cannot be supplied to the worked part and the workability easily drops due to clogging.

The third is the problem of the heating method. For example, SiC is extremely good in heat conductivity and high in coolability. Therefore, even if heating the surface to a predetermined temperature, it is immediately cooled and falls in temperature and the softening effect due to heating is reduced.

The inventors studied in depth the method for solving the above problem.

As a result, regarding the first problem, they discovered that by increasing the speed of the grindstone to a high speed and, further, by cooling at a location different from the worked part, the problem can be solved. That is, the grindstone requires heat input from the heated worked material by heat conductivity, but by rotation at a high speed so that the heat input becomes smaller than the heat resistance temperature of the abrasive, the contact time between the worked material and the abrasive is shortened. Further, the temperature which had risen due to the heat input due to contact is returned to ordinary temperature by cooling at a position different from the worked part.

Regarding the second problem, the inventors discovered that by performing the washing at a different position from the worked part, the problem can be solved. As explained above, the abrasive is cooled at a different position from the worked part and is returned to ordinary temperature, so the abrasive can be washed. For the washing method, vapor washing, high pressure washing, washing by an ultrasonically treated working solution, etc. may be used.

Regarding the third problem, the inventors discovered that the problem could be solved by making the heating source for heating the worked material a temperature higher than a predetermined temperature, considering the heat loss due to heat diffusion and heat conduction, and making the worked part a predetermined temperature.

The present invention is a method, based on the above knowledge, of working a high hardness material by making the high hardness material touch a polishing tool while making the polishing tool rotate, which method is comprised of

a step of heating the high hardness material to a temperature higher than a predetermined working temperature at a position different from a worked part so as to become the working temperature at that worked part and thereby form a softened layer with a hardness lower than a surface layer of the high hardness material, a step of working the high hardness material by making the high hardness material touch the polishing tool while making the polishing tool rotate at a speed such that the temperature of the polishing tool which rises due to heat input from the high hardness material at worked part to the polishing tool becomes no more than a heat resistance temperature of the polishing tool, a step of cooling the polishing tool at a position which is different from the worked part, and a step of washing the polishing tool at a position which is different from the worked part.

According to the present invention, it becomes possible to eliminate the problems of the heat resistance and clogging of a grindstone and work a worked material with a high hardness by heating it to a high temperature for softening, so the working time can be shortened and the working efficiency of the high hardness material can be improved.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects and features of the present invention will become clearer from the following description of the preferred embodiments given with reference to the attached drawings, wherein:

FIG. 1A is a top schematic view of a working apparatus seen from above which shows an outline of a working method of the present invention.

FIG. 1B is a cross-sectional schematic view of a cross-section A-A′ of FIG. 1A.

DESCRIPTION OF PREFERRED EMBODIMENTS

Below, one example of an embodiment of the present invention will be explained in detail with reference to the drawings while using a worked material of a SiC single crystal wafer and an abrasive of the grindstone of cBN.

FIG. 1A is a schematic view of a working apparatus seen from above which shows an outline of a working method of the present invention. FIG. 1B is a schematic view of a cross-section A-AT of FIG. 1A. The working apparatus (1) which is shown in FIG. 1A is provided with a rotary stage (16), a polishing tool (grindstone) (21), a polishing wheel (26), a rotary shaft (27), a heating device (31), a cooling/washing device (41), a temperature detection device (51), a working resistance detecting device (61), and a control device (not shown).

The worked material (11) constituted by the SiC single crystal wafer is fastened to rotary stage (16) which has a not shown rotation mechanism. The worked material (11) is worked by making the worked material (11) which is fastened to the rotary stage (16) touch a rotating polishing tool (21).

The polishing tool (21) is, for example, comprised by using a grindstone. By making it touch the worked material (11) while rotating, it works the surface of the worked material (11). In FIG. 1A, the worked material (11) and grindstone (21) are both shown so as to rotate counterclockwise, but the direction of rotation is not limited to this.

The heating device (13) is provided at a position different from the worked part (12) where the worked material (11) contacts the rotating grindstone (21). By heating the worked material (11) to become a predetermined working temperature at the worked part (12), the surface layer of the worked material (11) is formed by a softened layer with a lower hardness. The heating source constituted by the heating device (31) is preferably arranged right before the worked part (12) in the direction of rotation of the worked material (11). That is, the worked material (11) which is heated by the heating device (31) is preferably arranged at a position where it is worked right after being heated.

When the abrasive of the grindstone (21) is cBN, since cBN has a heat resistance temperature of 1280° C., the output of the heating device (31), heating area, and distance from the heating position to the worked part (12) are set so that the temperature of the SiC wafer of the worked part (12) becomes 1200° C. or so.

SiC is a material which has a good heat conductivity and even if heated is easily cooled, so the heating by the heating device (31) is set to give a higher temperature then the 1200° C. of SiC at the position of the heating device (31).

The hardness of the SiC is about Hv2500, but by heating to 1200° C. or so, a softened layer with a dropped hardness is formed at the surface layer. This is believed to be due to the fact that heating eases the molecular cohesion.

As the heating method which is used in the heating device (31), a method which enables heating up to 3000° C. or so, for example, dielectric heating, induction heating, and radiant heating is suitable. In the case of radiant heating, it is preferable to use a heating-use hot wall made of carbon with a heat resistance temperature of 3400° C.

When the worked material (11) is SiC, the coolability is extremely high. When heating locally, raising the temperature to a predetermined temperature is difficult, so the heating device (31) preferably has a certain heating area.

By using the heating device (31) to heat the worked material (11) and lower the hardness of the surface layer, the working can be performed at a high speed and the efficiency of working can be made higher.

The worked material (11) and grindstone (21) are made to rotate at a high speed so that the temperature rise caused by the heat input from the worked material which was heated to a high temperature does not cause the grindstone (21) to soften etc.

The grindstone (21) is cooled and washed at a position different from the worked part (12) by the cooling/washing device (41). As the cooling/washing means, vapor washing/cooling, high pressure washing, washing/cooling by an ultrasonically treated coolant, etc. may be used.

The temperature detection device (51) is placed above the worked material (11). For the temperature detection device (51), for example, a radiant thermometer etc. may be used. The temperature detection device (51) detects the temperature of the worked material (11) at the worked part (12) and sends the detected temperature to the control device.

For the working resistance detecting device (61), for example, a torque meter etc. may be used. The working resistance detecting device (61) detects a resistance value which is transmitted through the grindstone (21) to the polishing wheel (26) or rotary shaft (27) which is making the grindstone (21) rotate when the grindstone (21) contacts the worked material (11) and/or detects the resistance value which is transmitted through the worked material (11) to the rotary stage (16) and sends the detected resistance value to the control device.

The control device controls the output of the heating device (31) in accordance with the temperature which is detected by the temperature detection device (51) and the working resistance which is detected by the working resistance detecting device (61). Specifically, when the detected temperature is lower than the threshold value which was set in advance or the working resistance is higher than the threshold value which was set in advance, the output of the heating device (31) is enlarged and the amount of heat which is given to the worked material (11) is increased.

The temperature detection device (51), working resistance detecting device (61), and control device do not necessarily have to be provided, but by providing these devices and suitably controlling the heating device (31), it becomes possible to reliably soften a desired region of the worked material (11), so it becomes possible to more efficiently work a high hardness material.

While the invention has been described by reference to specific embodiments chosen for purposes of illustration, it should be apparent that numerous modifications could be made thereto by those skilled in the art without departing from the basic concept and scope of the invention. 

1. A method of working a high hardness material by making the high hardness material touch a rotating polishing tool while making the high hardness material rotating comprising the steps of: heating the high hardness material to a temperature higher than a predetermined working temperature at a position different from a worked part so as to become the working temperature at that worked part and thereby forming a softened layer with a hardness lower than a surface layer of the high hardness material; working the high hardness material by making the polishing tool rotate at a speed such that the temperature of the polishing tool which rises due to heat input from the high hardness material at worked part to the polishing tool becomes no more than a heat resistance temperature of the polishing tool while making the high hardness material touch the polishing tool; cooling the polishing tool at a position which is different from the worked part; and washing the polishing tool at a position which is different from the worked part.
 2. The method of working a high hardness material as set forth in claim 1 characterized by detecting a temperature and working resistance of the worked part of the high hardness material and controlling an output of a heating source which heats the high hardness material in accordance with the detected value.
 3. The method of working a high hardness material as set forth in claim 1 wherein the heating of the high hardness material is dielectric heating, induction heating, or radiant heating.
 4. The method of working a high hardness material as set forth in claim 1 wherein the high hardness material is SiC and in that the polishing tool is cBN.
 5. A working apparatus which works a high hardness material by making it rotate while making it abut against a rotating polishing tool comprising: a heating device which heats the high hardness material at a position different from a worked part; a cooling device which cools the polishing tool at a position which is different from the worked part; and a washing device which washes the polishing tool at a position which is different from the worked part. 